'\" et
.TH RECVMSG "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
recvmsg
\(em receive a message from a socket
.SH SYNOPSIS
.LP
.nf
#include <sys/socket.h>
.P
ssize_t recvmsg(int \fIsocket\fP, struct msghdr *\fImessage\fP, int \fIflags\fP);
.fi
.SH DESCRIPTION
The
\fIrecvmsg\fR()
function shall receive a message from a connection-mode or
connectionless-mode socket. It is normally used with
connectionless-mode sockets because it permits the application to
retrieve the source address of received data.
.P
The
\fIrecvmsg\fR()
function takes the following arguments:
.IP "\fIsocket\fR" 12
Specifies the socket file descriptor.
.IP "\fImessage\fR" 12
Points to a
.BR msghdr
structure, containing both the buffer to store the source address and
the buffers for the incoming message. The length and format of the
address depend on the address family of the socket. The
.IR msg_flags
member is ignored on input, but may contain meaningful values on
output.
.IP "\fIflags\fR" 12
Specifies the type of message reception. Values of this argument are
formed by logically OR'ing zero or more of the following values:
.RS 12 
.IP MSG_OOB 12
Requests out-of-band data. The significance and semantics of
out-of-band data are protocol-specific.
.IP MSG_PEEK 12
Peeks at the incoming message.
.IP MSG_WAITALL 12
On SOCK_STREAM sockets this requests that the function block until the
full amount of data can be returned. The function may return the
smaller amount of data if the socket is a message-based socket, if a
signal is caught, if the connection is terminated, if MSG_PEEK was
specified, or if an error is pending for the socket.
.RE
.P
The
\fIrecvmsg\fR()
function shall receive messages from unconnected or connected
sockets and shall return the length of the message.
.P
The
\fIrecvmsg\fR()
function shall return the total length of the message. For
message-based sockets, such as SOCK_DGRAM and SOCK_SEQPACKET, the
entire message shall be read in a single operation. If a message is too
long to fit in the supplied buffers, and MSG_PEEK is not set in the
.IR flags
argument, the excess bytes shall be discarded, and MSG_TRUNC shall be
set in the
.IR msg_flags
member of the
.BR msghdr
structure. For stream-based sockets, such as SOCK_STREAM, message
boundaries shall be ignored. In this case, data shall be returned to
the user as soon as it becomes available, and no data shall be
discarded.
.P
If the MSG_WAITALL flag is not set, data shall be returned only up to
the end of the first message.
.P
If no messages are available at the socket and O_NONBLOCK is not set on
the socket's file descriptor,
\fIrecvmsg\fR()
shall block until a message arrives. If no messages are available at
the socket and O_NONBLOCK is set on the socket's file descriptor, the
\fIrecvmsg\fR()
function shall fail and set
.IR errno
to
.BR [EAGAIN] 
or
.BR [EWOULDBLOCK] .
.P
In the
.BR msghdr
structure, the
.IR msg_name
member may be a null pointer if the source address is not required.
Otherwise, if the socket is unconnected, the
.IR msg_name
member points to a
.BR sockaddr
structure in which the source address is to be stored, and the
.IR msg_namelen
member on input specifies the length of the supplied
.BR sockaddr
structure and on output specifies the length of the stored address.
If the actual length of the address is greater than the length of the
supplied
.BR sockaddr
structure, the stored address shall be truncated. If the socket is
connected, the
.IR msg_name
and
.IR msg_namelen
members shall be ignored. The
.IR msg_iov
and
.IR msg_iovlen
fields are used to specify where the received data shall be stored.
The
.IR msg_iov
member points to an array of
.BR iovec
structures; the
.IR msg_iovlen
member shall be set to the dimension of this array. In each
.BR iovec
structure, the
.IR iov_base
field specifies a storage area and the
.IR iov_len
field gives its size in bytes. Each storage area indicated by
.IR msg_iov
is filled with received data in turn until all of the received data is
stored or all of the areas have been filled.
.P
Upon successful completion, the
.IR msg_flags
member of the message header shall be the bitwise-inclusive OR of all
of the following flags that indicate conditions detected for the
received message:
.IP MSG_EOR 12
End-of-record was received (if supported by the protocol).
.IP MSG_OOB 12
Out-of-band data was received.
.IP MSG_TRUNC 12
Normal data was truncated.
.IP MSG_CTRUNC 12
Control data was truncated.
.SH "RETURN VALUE"
Upon successful completion,
\fIrecvmsg\fR()
shall return the length of the message in bytes. If no messages are
available to be received and the peer has performed an orderly
shutdown,
\fIrecvmsg\fR()
shall return 0. Otherwise, \-1 shall be returned and
.IR errno
set to indicate the error.
.SH ERRORS
The
\fIrecvmsg\fR()
function shall fail if:
.TP
.BR EAGAIN " or " EWOULDBLOCK
.br
The socket's file descriptor is marked O_NONBLOCK and no data is
waiting to be received; or MSG_OOB is set and no out-of-band data is
available and either the socket's file descriptor is marked O_NONBLOCK
or the socket does not support blocking to await out-of-band data.
.TP
.BR EBADF
The
.IR socket
argument is not a valid open file descriptor.
.TP
.BR ECONNRESET
A connection was forcibly closed by a peer.
.TP
.BR EINTR
This function was interrupted by a signal before any data was
available.
.TP
.BR EINVAL
The sum of the
.IR iov_len
values overflows a
.BR ssize_t ,
or the MSG_OOB flag is set and no out-of-band data is available.
.TP
.BR EMSGSIZE
The
.IR msg_iovlen
member of the
.BR msghdr
structure pointed to by
.IR message
is less than or equal to 0, or is greater than
{IOV_MAX}.
.TP
.BR ENOTCONN
A receive is attempted on a connection-mode socket that is not
connected.
.TP
.BR ENOTSOCK
The
.IR socket
argument does not refer to a socket.
.TP
.BR EOPNOTSUPP
The specified flags are not supported for this socket type.
.TP
.BR ETIMEDOUT
The connection timed out during connection establishment, or due to a
transmission timeout on active connection.
.P
The
\fIrecvmsg\fR()
function may fail if:
.TP
.BR EIO
An I/O error occurred while reading from or writing to the file
system.
.TP
.BR ENOBUFS
Insufficient resources were available in the system to perform the
operation.
.TP
.BR ENOMEM
Insufficient memory was available to fulfill the request.
.LP
.IR "The following sections are informative."
.SH "EXAMPLES"
None.
.SH "APPLICATION USAGE"
The
\fIselect\fR()
and
\fIpoll\fR()
functions can be used to determine when data is available to be
received.
.SH "RATIONALE"
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIpoll\fR\^(\|)",
.IR "\fIpselect\fR\^(\|)",
.IR "\fIrecv\fR\^(\|)",
.IR "\fIrecvfrom\fR\^(\|)",
.IR "\fIsend\fR\^(\|)",
.IR "\fIsendmsg\fR\^(\|)",
.IR "\fIsendto\fR\^(\|)",
.IR "\fIshutdown\fR\^(\|)",
.IR "\fIsocket\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<sys_socket.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
